Interleukin 9 (IL-9) was initially discovered as a 40KD glycoprotein with T-cell growth factor activity in mice (1). Human IL-9 gene was identified based on its remarkable homogeneity with the gene of mouse IL-9 (2) and was mapped to chromosome 5. Human IL-9 has been demonstrated to be a stimulator for hematopoiesis and be associated with human malignant lymphoma (3, 4, 5, 6). In addition, IL-9 and IL-9 receptor expression was found to be implicated with autoimmune and allergic diseases (7, 8). IL-9 has been studied as a therapeutic target for asthma (7, 9) because of its implication with the disease. IL-9 can be produced by Th2 cells under the stimulation of TGF-b and IL-4 and has been shown to play a role in the differentiation of Th17 cells and Treg function (10, 11). In recent years, IL-9 has received renewed attention because a unique type of innate lymphoid cells has been identified as the main cell type that expresses IL-9 in vivo and the IL-9 secreting cells present in skin appear to be mediate inhibition of melanoma growth in mice (12,13,14,15). The same IL-9 secreting cells have been identified in human and the cells are currently evaluated as a new tumour treatment strategy. This IL-9 ELISA is a ready-to-use 3.5-hour solid phase immunoassay readily capable of measuring IL-9 levels in serum, plasma, cell culture supernatant, and other biological fluids in the range of 0 to 50 pg/mL. This assay has shown no cross-reactivity with other cytokines tested, and is expected to be used effectively for further investigations into the relationship between IL-9 and the various conditions mentioned.